Fix preview command key bindings

[EFanZh]

Preview commands should only be handled in Markdown documents.

[yzhang-gh]

Thanks for your contribution.

However, things are a bit more complex. These keybindings are designed to "toggle" preview (open or close). When the active tab is the Markdown preview, we cannot require editorLangId == markdown.

[EFanZh]

How about using view == 'markdown.preview' (See here)?

The problem is that if I press Ctrl + Shift + V immediately after opening VS Code, the editor reports the following error:

command 'markdown.extension.togglePreview' not found

I think this is because the extension hasn’t been activated.

[yzhang-gh]

Thanks for the information.

It seems that markdown.preview is not a context key.
There are also many undocumented context keys, which are usually set by
vscode.commands.executeCommand('setContext', key, value);

These keys can be found in the vscode source code. I tried to find but nothing. I am not sure whether we have a proper context key now.

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If we still cannot find a good solution, as a workaround, you can delete the conflict keybinding (open keybinding page, search for that, right-click and delete).